In recent years, in order to decrease the size and increase the performance of industrial equipment and home-use apparatuses, the use of devices incorporating semiconductor applications has been expanding. The power rectifier circuit and the phase control circuit built in such devices use a capacitor. The large pulse-like input current for charging the capacitor increases the high-harmonic current and voltage distortion in the transmission line and the power equipment. The devices are thus adversely affected and the power factor thereof is reduced considerably. Various methods have been suggested for suppressing the high-harmonic current and improving the power factor. Of all these methods, a comparatively simple and low-cost method is closely watched in which a choke coil is inserted in series (in normal mode) in the AC line.
A conventional choke coil for preventing harmonic distortions shown in FIGS. 32 to 34 is well known. FIGS. 32 to 34 show an exploded perspective view, a sectional view and an equivalent circuit respectively of a conventional choke coil used for preventing harmonic distortions.
In FIGS. 32 to 34, numeral 58 designates a U-shaped closed-circuit magnetic core made of a ferrite material, numeral 59 an EI-shaped closed-circuit magnetic core made of silicon steel sheets, numeral 60 a bobbin, numerals 61, 62 coils, numeral 63 a resin case, numeral 64 a shield case, numeral 65 a casting resin, numeral 66 partitioning flanges, numeral 67 a magnetic gap, character "C" a common-mode choke coil section and character "N" a normal-mode choke coil section.
The above-mentioned choke coil for preventing harmonic distortions is completed by combining the U-shaped closed-circuit magnetic core 58 of a ferrite material and the EI-shaped closed-circuit magnetic core 59 of silicon steel sheets, with the coils 61, 62 having the same number of turns wound on the bobbin 60 partitioned by the partitioning flange 66 in such a manner as to cover two magnetic cores 58, 59. In this configuration, as shown by the equivalent circuit of FIG. 34, two different closed-circuit magnetic cores 58, 59 constitute different magnetic circuits, and the normal-mode choke coil section "N" is configured mainly of the EI-shaped closed-circuit magnetic core 59 of silicon steel sheets while the common-mode choke coil section "C" is constructed mainly of the U-shaped closed-circuit magnetic core 58 of a ferrite material. The magnetic gap 67 provided on the middle limb of the EI-shaped magnetic core 59 made of silicon steel sheets is for improving the magnetic saturation characteristic of the normal-mode choke coil section "N".
For a choke coil for preventing harmonic distortions, the important problem is generally how to secure a very large inductance value on the order of several mH in normal mode and reduce the package space and weight at the same time. The conventional choke coil for preventing harmonic distortions shown in FIG. 32 can secure a normal-mode inductance value required for preventing harmonic distortions, while at the same time having the function of a common-mode choke coil. Therefore, prevention of both harmonic distortions and EMI is possible, and also the common-mode choke coil thus far arranged in the filter block of the power circuit can be eliminated, thereby leading to the additional advantage of reducing the package space.
In the conventional choke coil for preventing harmonic distortions, however, due to the configuration of the magnetic circuit thereof, the coil 61 and the coil 62 cannot be arranged closely to each other and are separated by the width of the middle limb of the EI-shaped magnetic core 59 of silicon steel sheets. As a result, the coupling coefficient between the coils 61 and 62 of the common-mode choke coil section "C" is reduced, so that the magnetic core 58 of a ferrite material is liable to be magnetically saturated. It is thus necessary to select a material of a high saturation flux density for the magnetic core 58. Generally, materials of a high saturation flux density have a low magnetic permeability, leading to the disadvantage of an increased size of the common-mode choke coil section "C". Also, in the normal-mode choke coil section "N", a great amount of leakage fluxes are generated from the magnetic gap 67 provided on the middle limb of the EI-shaped magnetic core 59 of silicon steel sheets, thereby posing the problem of an adverse effect on the other parts.